LEHILLIER/MANKATO

EPA ID# MND980792469
Last Updated: November, 2014

Site Description

The LeHillier/Mankato site, located in Blue Earth County, Minnesota, is a 1,000-acre site where uncontrolled dumping occurred until 1972. In the early 1980s, the Minnesota Pollution Control Agency (MPCA) confirmed volatile organic compound (VOC) contamination of groundwater in the area by sampling domestic wells. A number of small dumps were known to exist in the area, but a specific source of the contamination was not found. Several residential wells were impacted, and one of the municipal wells for Mankato, a city of 30,000 people, is directly downgradient and less than one-quarter mile from the VOC plume. The surficial aquifer, which is the impacted aquifer at the site, is the drinking water source for Mankato. No contamination has been detected to date in the municipal well near the site. A new municipal well was recently constructed approximately one-quarter mile to the north of the contamination. Both municipal wells draw from the shallow aquifer.

Site Responsibility

The site is being addressed through federal and state actions.

Threats and Contaminants

Prior to site cleanup actions, the groundwater was contaminated with VOCs. Residual groundwater VOC contamination remains at the site. Drinking or coming into contact with VOC-contaminated groundwater may pose a health risk. Currently there are no known users of the groundwater as a drinking water source. To achieve an acceptable level of risk, the cleanup level for the main VOC contaminant, trichloroethylene (TCE), was set at 2.8 micrograms per liter in the original 1985 cleanup decision by the EPA. This cleanup level was revised to 5.0 micrograms per liter in a August 2008 cleanup decision document revision, making the TCE groundwater cleanup level consistent with the federal drinking water standard. As of May of 2013, TCE levels remain just above the MCLs and groundwater monitoring is ongoing.

Cleanup Progress

In 1983 LeHillier residents were provided with bottled water, and in 1985 a municipal water supply system was constructed for the residents of LeHillier through a Housing and Urban Development grant. EPA conducted a remedial investigation (a study of the nature and extent of site contamination) and a feasibility study (a study of the site cleanup alternatives) in the mid-1980s and published a Record of Decision (ROD) for the site in 1985. The 1985 ROD called for construction of a groundwater extraction system and air stripping treatment unit and proper abandonment of 83 private wells. In 1989, MPCA constructed the groundwater treatment system under a Cooperative Agreement with EPA. In 2003, the LeHillier/Mankato site was deleted from the Minnesota Permanent List of Priorities (PLP).

A third five-year review of the remedy, completed in September 2006, found the remedy to be protective of human health and the environment. There is no known use of groundwater in the area for drinking, and the extent of TCE-contaminated groundwater is limited to an area one-quarter mile away from the municipal wells. Because of the low levels of TCE remaining in groundwater and the limited area of contamination, in 2007 the air stripper was dismantled. An Explanation of Significant Differences (ESD) was signed by EPA in August 2008 to document the dismantling of the groundwater treatment system and to establish a revised cleanup level for TCE - 5 micrograms per liter, the federal maximum contaminant level (MCL) drinking water standard. The ESD also documented several other modifications, such as the implementation of an ordinance in 1987 by South Bend Township that restricted the use of groundwater in the area. This ordinance was enacted voluntarily by the township even though institutional controls were not required in the 1985 ROD. Currently, an updated version of the ordinance is under review by South Bend Township.

The fourth Five-Year Review determined that the site remains protective in the short-term. TCE contamination in groundwater continues to be at low levels. Results from recent VOC analysis of water from the City of Mankato municipal wells show no detectable concentrations of VOCs.